Backup material for small bore drilling

ABSTRACT

A backup material for small bore drilling in hard .[.crystalline.]. materials, such as multilayer circuit boards containing polyimide/glass, polyimide kevlar or modified epoxy BT/glass, includes at least one layer of hard crystalline material. Standard LCOA material with an additional layer selected from the group comprising polyimides, fluorocarbons and high temperature polycarbonates has proven successful.

This invention relates to a backup or substrate for drilling small holesin polyimide and modified epoxy materials.

Multilayer printed circuit boards are typically constructed of aninsulative layer separating conductive layers, such as copper. Standardmultilayer boards, having 2 to 20 layers, range in thickness from 0.015to about 0.125 inches each. Heretofore, it was possible to successfullydrill holes down to about 0.006 inches in diameter in printed circuitboards constructed of epoxy glass using backup materials such as LCOA, amaterial commercially available from Laminating Company of America ofEscondido, Calif. However, more recently printed circuit boards havebeen constructed of polyimide/glass, polyimide/kevlar and modified epoxyBT/glass materials. These particular materials are .[.hard crystallinestructures,.]. .Iadd.hard structures, characterized by a rough,grain-like appearance when broken or fractured, .Iaddend.such thatdrilling in them is more difficult than in prior materials. It has notbeen possible to drill holes at the small diameters previously achievedusing existing backup materials, such as the LCOA material.Particularly, it has been found that attempts to drill apertures smallerthan about 0.0165 inches in .[.hard crystalline.]. .Iadd.such hard.Iaddend.structures, having thicknesses greater than about 0.065 inches,results in a high incidence of drill bit breakage. Further, it has beenfound impractical to attempt to drill holes in such materials belowabout 0.0135 inches in diameter. In particular, drill bit breakageoccurs largely due to compaction of the .[.hard crystalline.]. materialin the flutes of the bit, which material "binds" against the wall of theaperture during bit retraction, causing bit breakage.

I have found, however, that if the backup material also includes a.[.hard crystalline material,.]. .Iadd.hard material, characterized by arough, grain-like appearance when broken or fractured, .Iaddend.theincidence of drill bit breakage is significantly reduced. Without beingbound to any theory, it is theorized that when drilling small bores in.[.hard crystalline.]. .Iadd.such hard .Iaddend.boards (polyimide andmodified epoxy BT glass), the .[.hard crystals.]. .Iadd.granules.Iaddend.(polyimide and epoxy .[.crystals.]. .Iadd.granules.Iaddend.)pack in the flutes of the drill bit. As the drill bit is retracted, the.[.crystals.]. .Iadd.granules .Iaddend.impacted in the flutes bindagainst the side walls of the bore, and the stress of retraction breaksthe drill bit. However, when using a backup material containing a.[.hard crystalline.]. material, such as polyimide, the flutes arecleaned prior to retraction. When the drill bit enters the polyimide andacrylic of the back-up material, the polyimide remains in a one piecesliver, which can withstand extremely high temperatures and thus remaina "slippery" lubricant to the flutes. The sliver of polyimide from thebackup material thus forces the .[.crystalline.]. .Iadd.granular.Iaddend.debris out of the flute prior to retraction of the bit.

It is an object of the present to provide a backup material for drillingsmall holes in .[.hard crystalline structures.]. .Iadd.hard structuressuch as polyimide.Iaddend..

Another object of the present invention is the provision of a .[.hardcrystalline.]. .Iadd.hard.Iaddend., or polyimide, layer on conventionalbackup materials which have the effect of cleaning the flute of drillbits prior to retraction of the bit.

The above and other features of this invention will fully understoodfrom the following detailed description and the accompanying drawings,in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a section view of a backup material in accordance with theprior art; and

FIG. 2 is a section view of a backup material for drilling small boardholes in .[.hard crystalline.]. .Iadd.hard .Iaddend.structures inaccordance with the presently preferred embodiment of the presentinvention.

Referring to FIG. 1, there is illustrated a section view of a backupmaterial for small bore drilling in accordance with prior arttechniques. The particular material illustrated in FIG. 1 is known asLCOA backup material and consists of a layer 10 of compressed wood chipsor sawdust in a bonding agent with aluminum foil layers 12 and 14adhered to both sides thereof by an adhesive layers 16 and 18. As shownin FIG. 2, the present invention, in its preferred form, consists of thestandard LCOA material illustrated in FIG. 1 with the addition of layers20 and 22, consisting of 0.002-inch thick layer of polyimide,commercially available from DuPont as Kapton or Kapton H. The polyimidelayers 20 and 22 are adhered to aluminum layers 12 and 14 by a0.001-inch thick layer of acrylic adhesive, commercially available fromOake or Fortin companies. Conveniently, the polyimide layers may beapplied to the aluminum with the acrylic adhesive in a heated lamination(rolling) process.

Tests of the present invention have been conducted. In a first test,standard LCOA backup material was used with a circuit board having tenlayers of polyimide and a thickness of 0.70 inches. In this first test,600 bores were drilled using drill bits having a diameter of 0.0135inches at a spindle speed of 80,000 RPM. With a drill feed of 40 inchesper minute, an average of one broken bit occurred in each 650 bores. Ina second test, the same conditions as the first test were used, exceptthe drill bit feed rate was increased to 60 inches per minute. Nearlyconstant bit breakage occurred in the second test. In similar testsusing smaller drill bits of the order of 0.012 inches, 0.011 inches, and0.010 inches at spindle speeds of 80,000 RPMs and drill feed of 50inches per minute, broken bits occurred in approximately each 20 bores.

In another set of tests employing backup material in accordance with thepresently preferred embodiment of the present invention with polyimideadhered to the LCOA material with an acrylic adhesive, at drill feedsranging between 30 and 100 inches per minute, and spindle speeds of80,000 RPM, drilling the same type of boards as in the first test, nobroken bits occurred in 1,500 bores using 0.0135-inch diameter bits, nobroken bits occurred in 1,500 bores with a drill size of 0.011-inch, andno broken bits occurred in 1,000 bores using drill sizes of 0.010inches.

Testing on printed circuit boards containing copper layers resulted inno broken bits in over 50,000 bores. Micro sectioning of multilayercircuit boards drilled with backup material in accordance with thepresent invention showed no degradation in hole quality.

The present invention thus provides an effective material for backup ofsmall bore drilling in .[.hard crystalline.]. .Iadd.hard.Iaddend.structures. Particularly, the material has proven useful inconnection with drilling materials constructed of polyimide glass,polyimide-kevlar and modified epoxy BT-glass material. In addition,although polyimide material is disclosed as being the preferred materialto adhered by acrylics to LCOA back-up material, it has also been foundthat flurocarbons, such as Teflon, and high temperature polycarbonatesare also useful in place of polyimide materials.

This invention is not to be limited by the embodiment shown in thedrawing and described in the description, which is given by way ofexample and not of limitation, but only in accordance with the scope ofthe appended claims.

What is claimed is:
 1. In a backup sheet for backing a work sheet duringa small bore drilling process, said work sheet containing at least onelayer of .[.hard crystalline.]. .Iadd.hard .Iaddend.material selectedfrom the group comprising polyimide/glass, polyimide/kevlar and modifiedepoxy BT/glass, said backup sheet having a core, the improvementcomprising an additional layer of .[.hard crystalline.]. .Iadd.hard.Iaddend.material selected from the group comprising polyimides,fluorocarbons and high temperature polycarbonates fixed to oppositesides of said core.
 2. A backup sheet according to claim 1 wherein saidadditional layer consists of a layer of polyimide of the order of about0.002 inches thick fixed to said core with a layer of acrylic adhesiveof the order of 0.001 inches thick.
 3. A backup sheet for backing a worksheet during a small bore drilling process, said work sheet containingat least one layer of .[.hard crystalline.]. .Iadd.hard.Iaddend.material, said backup sheet comprising a core constructed of acompressed organic material and a binder, said organic material beingselected from the group comprising wood chips and sawdust, said corehaving opposite planar surfaces; a first aluminum layer adhered to oneof said planar surfaces of said core and a second aluminum layer adheredto the other of said planar surfaces of said core; and a first layer of.[.hard crystalline.]. .Iadd.hard .Iaddend.material adhered to saidfirst aluminum layer and a second layer of .[.hard crystalline.]..Iadd.hard .Iaddend.material adhered to said second aluminum layer, saidfirst and second layers of .[.hard crystalline.]. .Iadd.hard.Iaddend.material being selected from the group comprising polyimides,fluorocarbons and high temperature polycarbonates.
 4. A backup sheetaccording to claim 3 wherein said first and second layers of .[.hardcrystalline.]. .Iadd.hard .Iaddend.materials each consist of a layer ofpolyimide of the order of about 0.002 inches thick and are adhered tothe respective first and second aluminum layers with a layer of acrylicadhesive of the order of about 0.001 inches thick.